Apparatus and method for burning a combustible gas, and a heat exchanger for use in this apparatus

ABSTRACT

An apparatus functioning as a high-efficiency water heater. This apparatus passes a combustible mixture of gas and air from an impeller into the hollow interior of a cylindrical gas burner. The combustible mixture flows out through apertures in the cylindrical walls of the gas burner and is burned on a cylindrical flame holder as it emerges; the size of the apertures prevents flash-back of the flames of the combustible mixture into the gas burner. Heat is transferred from the flame to the walls of a cylindrical combustion chamber surrounding the gas burner and the combustion products generated pass to a heat exchanger comprising a vertical riser and a downwardly extending helical section which ensures maximum heat transfer to water in a tank surrounding the combustion chamber and heat exchanger. Hot water is discharged from the tank as needed for domestic uses and to another heat exchanger in an air heating duct.

This is a continuation-in-part of application Ser. No. 515,642, filedJuly 20, 1983, now U.S. Pat. No. 4,541,410.

FIELD OF THE INVENTION

This invention relates to an apparatus and method for burning acombustible gas and passing the gaseous mixture to a heat exchanger foreffecting heat exchange between the hot combustion products and aliquid.

BACKGROUND OF THE INVENTION

Because of the rapidly rising cost of natural gas, in recent years mucheffort has been directed to improving the efficiency of gas furnaces forresidential and other consumers. Most modern gas furnaces use electronicignition rather than a pilot light to avoid the inevitable waste of gaswhich a pilot light involves. Also, very high efficiency gas furnaceshave been introduced; some of these furnaces use a pulse system in whichpulses of a gas and air mixture are ignited within a combustion chamber,while others rely upon relatively complicated heat exchangers to extractthe maximum amount of heat from the combustion products produced byburning natural gas. The latest furnaces are considerably more efficientthan the older, pilot ignition gas furnaces; a typical pilot ignitiongas furnace might have a steady state efficiency of about 75% and aseasonal efficiency of about 65%, while replacing the pilot light witheither direct spark ignition of intermittent pilot ignition increasesthe seasonal efficiency to about 70%. Certain of the high-efficiency gasfurnaces previously mentioned have seasonal efficiencies of about 92-95%under specialized conditions.

Although much attention has thus been directed to improving theefficiency of gas furnaces, many residential gas consumers have both agas furnace for space heating and a gas water heater, and relativelylittle research appears to have been performed to improve the efficiencyof gas water heaters. The design of residential gas water heaters haschanged relatively little over the last few years. Most residential gaswater heaters comprise a cylindrical water tank provided with aninsulating jacket and a gas burner which impinges upon the base of thewater tank; to allow escape of combustion products produced by theburning gas, a vertical conduit carrying the exhaust products extendsvertically upwardly along the axis of the cylindrical tank, thisvertical conduit serves to effect additional heat exchange between thecombustion products and the water in the tank. The steady state andseasonal efficiencies of such gas water heaters are only about 70% and55%, respectively, considerably lower than those of the high-efficiencytypes of gas furnaces previously described. Thus, the overall efficiencyof gas consumption by most consumers is markedly diminished by therelative inefficiency of conventional gas water heaters. In fact, whenappropriate weighting is given to the relative amounts of gas used by aheat efficient furnace and a conventional water heater in a typicalhousehold, the combined seasonal efficiency of gas usage is only about55%. There is thus a need to improve the efficiency of gas water heatersin order to increase the overall efficiency of gas use.

The use of separate furnace and water heating units has otherdisadvantages in addition to the relatively low overall efficiency ofgas use. The separate furnace and water heater require a relativelylarge amount of space and also require two separate pilot lights orother ignition systems, and separate gas lines, thus increasinginstallation costs.

In order to overcome the aforementioned disadvantages of separate gasfurnaces and gas water heating units, it is desirable to provide asingle unit which functions both as a gas furnace for space heating andas a gas water heater. This invention provides an apparatus which canfunction both as a gas furnace and as a gas water heater and alsoprovides a method for burning a combustible gas which enables such acombined gas furnace and water heater to achieve higher efficiency.Finally, this invention provides a heat exchanger effecting heatexchange between a hot gas and a liquid which can be used in theaforementioned combined gas furnace and water heater.

SUMMARY OF THE INVENTION

This invention provides apparatus for burning a combustible gascomprising a housing having walls defining a liquid chamber capable ofholding liquid and a combustion chamber member disposed within theliquid chamber and having liquid-impervious walls defining a combustionchamber. Within the combustion chamber is disposed a gas burner havingwalls defining an internal chamber and apertures passing through thesewalls, thus establishing fluid communication between the internalchamber of the gas burner and the combustion chamber outside the gasburner. Finally, the apparatus comprises an impeller for passing acombustible mixture of the combustible gas and an oxygen-containing gasunder pressure into the internal chamber of the gas burner. Theapertures in the gas burner are so sized that combustion of thecombustible mixture within the combustion chamber outside the gas burnerwill not cause ignition of the combustible mixture within this internalchamber.

The invention also provides a method for burning a combustible gascomprising mixing the combustible gas with an oxygen-containing gas toform a combustible mixture capable of supporting combustion withoutaddition of any further oxygen-containing gas, passing this combustiblemixture under pressure into an internal chamber of a gas burner, thisgas burner having walls defining the internal chamber and aperturespassing through these walls, these apertures being so sized thatcombustion of the combustible mixture outside the gas burner will notcause ignition of the combustible mixture within the internal chamber ofthe gas burner, permitting the combustible mixture to pass through theseapertures, and burning the combustible mixture as it leaves theapertures.

Finally, the invention provides a heat exchanger for effecting heatexchange between a hot gas and a liquid, this heat exchanger comprisinga housing having walls defining a liquid chamber capable of holdingliquid and a gas conduit disposed within the liquid chamber. This gasconduit comprises a riser section capable of being connected to a sourceof hot gas and extending upwardly therefrom, and a substantially helicalsection extending downwardly from the upper end of the riser section.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a vertical sectional view of a water heater according to theinvention; and

FIG. 2 is a vertical sectional view of an alternative embodiment of thewater heater of FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION

The instant apparatus for burning combustible gas differs fromconventional gas furnaces and from conventional water heaters in themanner in which the combustible gas and oxygen-containing gas are mixed.(In the following description, the instant method and apparatus willnormally be described assuming that the oxygen-containing gas is air; iffor any reason it is desired to use a different oxygen-containing gas,references to such gas may of course be substituted for references toair.) In conventional gas furnaces pure gas flows out of one or more gasnozzles, thereby becoming mixed with air to form a combustible mixturewhich burns immediately adjacent the gas nozzle. In the operation of theinstant apparatus and in the instant method, the combustible gas and theair are pre-mixed to form a combustible mixture which is then forcedinto the internal chamber of the gas burner (in saying that, in theinstant method, the combustible mixture is passed "under pressure" intothe internal chamber of the gas burner, I mean only that the combustiblemixture enters this internal chamber at a pressure greater than thepressure existing immediately outside the gas burner, so that thecombustible mixture will flow out of the gas burner via the aperturestherein). The apertures in the gas burner are sized so that combustionof the gas around the gas burner will not cause ignition of thecombustible mixture within the gas burner thus, preventing the occurenceof any flashback and/or explosion within the burner or impeller. Thepremixing of gas and air achieved in my apparatus and method enables thegas/air ratio to be precisely controlled, in contrast to a conventionalfurnace in which, because only combustible gas flows from the nozzle,only limited control can be exercised over the gas/air ratio. Control ofthe gas/air ratio is important in achieving maximum efficiency of gasusage, since an excess of air over that required for combustion of thegas simply dilutes the combustion products, reducing the temperaturethereof and thus reducing the efficiency of heat exchange between thecombustion products and any heat exchanger which serves to remove heatfrom the combustion products and supply it to where it is needed. Thoseskilled in the art are aware that conventional gas furnaces and waterheaters draw past the gas nozzles considerably more air than is requiredfor proper combustion of the gas. It is critical to exercise effectivecontrol over the gas/air ratio in order to avoid dilution of thecombustion products by excess air, and lack of control is one of thefactors which reduces the efficiency of conventional gas furnaces andwater heaters. While I do not absolutely exclude the possibility thatthe design of the combustion chamber in my apparatus may allow for theentry of some additional air thereinto, in addition to the combustiblemixture issuing from the gas burner, I very much prefer that thecombustion chamber have no gas inlet other than the gas burner so thatall the air required for combustion is mixed with the combustible gasprior to its entry into the gas burner.

As already mentioned, in the instant apparatus the impeller forces amixture of combustible gas and air into the gas burner. The admixing ofthe combustible gas and air may be effected either upstream ordownstream of the impeller; i.e. the combustible gas may be admixed withair upstream of the impeller and the resultant mixture passed throughthe impeller, or alternatively only air may pass through the impellerand admixing of the air with the combustible gas be effected between theimpeller and the gas burner. In the former case it is of courseessential that the impeller be of a type which will not generate sparksor the like capable of igniting the combustible mixture of gas and airpassing through it; in the latter case, no such restriction on the typeof impeller is required.

In theory the amount of air mixed with the combustible gas before entryinto the gas burner should be equal to the theoretical amount requiredfor complete combustion of the combustible gas. However, in practice itis desirable to provide a small excess of air in order to allow fortransient fluctuations in the gas/air ratio due to fluctuations in gaspressure and the like. Thus, the amount of air or otheroxygen-containing gas mixed with the combustible gas to form thecombustible mixture is desirably from 1.0 to about 1.2, and preferablyabout 1.1, times the amount of air or other oxygen-containing gasstoichiometrically required for complete combustion of the combustiblegas.

I have also discovered that the geometric shape of the gas burner isimportant in the instant apparatus and method. Very desirably, thecombustion chamber and the gas burner of my apparatus have substantiallythe form of a pair of co-axial cylinders and the apertures in the gasburner are disposed in the cylindrical wall of the gas burner, therebypermitting combustion of the combustible mixture on a cylindrical flamefront surrounding the gas burner. This cylindrical flame front allowsfor very good heat transmission from the flame to the cylindrical wallof the combustion chamber which, when the apparatus is in use, isimmersed in liquid held in the liquid chamber, and thus promotes veryefficient heat transfer from the flame to the liquid. Where thecylindrical burner has an end wall, this end wall may or may not beprovided with apertures i.e. the cylindrical burner may have aperturesin the side walls only or in both the side walls and the end wall. Othershapes of gas burner may also be employed; for example, the burner maybe conical, frustoconical or hemispherical or have the form of ahemisphere truncated by a plane parallel to its base. To ensure goodheat transfer to the walls of the combustion chamber, desirably thecombustion chamber has substantially the same form as the burner.However, it has been found that a cylindrical gas burner havingapertures only in the side walls is the most efficient form. It will beappreciated, of course, that the cylindrical flame front which can beachieved in the instant apparatus using a cylindrical gas burner cannotbe achieved by a conventional gas burner in which pure combustible gasissues from the nozzle, since the flame front produced by such a gasburner is always substantially conical.

The presently preferred embodiment of my apparatus will now bedescribed, though by way of illustration only, with reference to theaccompanying drawings. FIG. 1 shows an apparatus (generally designated10) which comprises a substantially cylindrical housing or water tank 12generally similar to the tank or liquid housing of a conventional waterheater except that it lacks the normal central, vertical exhaustconduit. As in a conventional gas water heater, the tank 12 issurrounded by an outer housing or cylinder 14 and an annular insulatingjacket 16 is disposed between the tank 12 and the outer cylinder 14 toreduce heat loss from the tank 12. The upper end of the outer cylinder14 is closed by an end plate 18; the insulating jacket 16 also fills thespace between the end plate 18 and the adjacent upper end wall of thetank 12 in order to reduce heat loss from the end wall of the tank 12.The lower end of the outer cylinder 14 extends downwardly beyond thelower end of the tank 12 and is closed by a base plate 20 which restsupon a floor or is supported by legs resting on the floor or othersuitable support surface and thus supports the entire apparatus. A pad22 of insulating material 16 is disposed above the base plate 20 toreduce heat loss via the base plate.

The lower end wall 24 of the tank 12 has fixed thereto a drain tube 26provided with a manually-operable valve 28 which is normally closed butwhich can be opened when it is desired to drain liquid from the tank 12.When the apparatus is in normal operation, the tank 12 is completelyfilled with water. The central portion of the lower-end wall 24 is flatand has a central circular aperture cut therein. This circular apertureis surrounded by a planer flange 30 which forms part of the lower-endwall of a combustion chamber member 32. This combustion chamber member32, lies wholly within the tank 12, is cylindro-conical in form havingthe circular plate 30 as part of its base, a cylindrical side wallsection 34 extending upwardly from around the periphery of the plate 30and an upper frusto-conical section 36 connected to the upper end of thecylindrical section 34. The various sections of the combustion chambermember 32 enclose a cylindro-conical combustion chamber 38. Within thiscombustion chamber 38 is a gas burner 40 having the form of a hollowcylinder closed at its upper end but open at its lower end. The gasburner 40 extends downwardly through the central circular aperture inthe lower-end wall 24 and has a flange 42 extending radially outwardlybelow the flange 30. The cylindrical wall of the gas burner 40 has amultitude of small apertures passing therethrough, thereby establishingfluid communication between the internal chamber within the hollowcylindrical gas burner 40 and the combustion chamber 38 lying outsidethe gas burner 40.

When the apparatus is in operation a combustible mixture of natural gasand air is fed under pressure to the internal chamber of the gas burner40 from an impeller 46 located between the lower end wall 24 of the tank12 and base plate 20. To secure a gas-tight connection between theimpeller 46 and the interior of the gas burner 40, the impeller 46 isprovided, at its outlet end, with a flange 47 for attachment to flange42 on the gas burner 40. The flange 30 is provided with fourequally-spaced threaded studs 44 which extend downwardly through bores(not shown) provided in the flanges 42 and 47 on the gas burner 40 andthe impeller 46. Obviously, more or fewer studs could be used. Nuts 45are screwed on to the lower ends of the studs 44, thereby securing theflanges 30, 42 and 47 together and establishing a gas-tight connectionbetween the impeller 46 and the interior of the gas burner 40. Gaskets(not shown) may of course be provided between adjacent pairs of theflanges 30, 42 and 47 to assist in obtaining good seals.

The impeller 46 may be of any convenient type and could be, for example,a vane pump or a fan. Indeed, the impeller could be attached at theexhaust end of the system rather than the inlet end.

Since, as described in more detail below, the impeller 46 is required topump a combustible mixture of natural gas and air, the impeller must inpractice be of a type which can pump such combustible gas without anyrisk of explosion, but it is believed that those skilled in the art willhave no difficulty in providing an impeller which meets theserequirements. (Alternatively, if mixing of the natural gas and air isarranged to take place downstream of the impeller, the impeller need notbe of any specific type). The impeller 46 is driven by an electric motor(not shown).

The impeller 46 draws its air from an air-inlet tube 48. The inlet end(not shown) of this tube 48 is preferably located externally of thebuilding in which the apparatus is installed in order that the air drawninto the impeller will be unconditioned, external air and not air whichis already conditioned. The portion of the tube 48 adjacent the mainpart of the apparatus 10 comprising a horizontal limb which terminatesabove the upper end plate 18, a vertical limb which extends verticallydownwardly between the cylindrical walls of the tank 12 and the outercylinder 14 within the insulating jacket 16, and a second horizontallimb which extends to the intake of the impeller 46. (The shape of thetube 48 is of course dictated solely by the type and position of airintake port with which the apparatus 10 is intended to be used and mayvary almost infinitely. Indeed, in principle tube 48 could terminateflush with the upper surface of the end plate 18 but such an arrangementwould have the obvious disadvantage of drawing combustion air fromwithin the building rather than using unheated outside air. Also, thetube 48 could be replaced by a simple horizontal tube extendingoutwardly from the impeller 46, i.e. by a horizontal tube which is ineffect an extension of the second horizontal limb shown. This placementof the tube 48 is designed so that none of the tube 48, which is made ofplastic, for example polyvinyl chloride (PVC), protrudes from thecylindrical wall of the outer cylinder 14, since any protruding parts ofthe tube would be susceptible to damage during transit (if in placeduring such transit), or damage after installation caused by, forexample, children, household pets, or careless handymen. Amanually-operable butterfly valve 50 may be disposed in the secondhorizontal limb of the tube 48 (but may alternatively be disposed in thevertical limb of the tube 48); this valve permits adjustment of thegas/air ratio in the mixture provided by the impeller 46 to the gasburner 40 and is accessible via a removable panel (not shown) in theouter cylinder 14; naturally, a corresponding aperture is provided inthe insulating jacket 16 adjacent this removable panel. The placement ofthe valve 50 is designed to permit ready adjustment of the valve 50 bythe person installing the apparatus but to render later access by otherpersons difficult in order to ensure, as far as possible, that untrainedpersons do not attempt to adjust this valve setting, since improperadjustment could result in incomplete combustion of the gas or otherundesirable consequences. Gas is supplied to the impeller via a gas line52 provided with a gas valve 53; the line 52 intersects the secondhorizontal limb of the tube 48 immediately adjacent the inlet of theimpeller 46.

The valve 50 in air feed line 48 is necessary where a positive pressuregas valve 53 is used. However, where a negative pressure gas valve 53 isused the air valve 50 may be eliminated as unnecessary. A negativepressure gas valve 53 senses the pressure in air feed line 48 andautomatically proportions the amount of gas supplied to the burner. Theobvious advantage of the negative pressure gas valve is that if the airflow is obstructed the volume of gas will automatically adjust to theobstructed condition.

Besides the gas burner 40, the combustion chamber 38 contains anelectrical ignition device 54 which can be of any conventional type andwhich is used in the conventional manner to ignite the combustiblegas/air mixture emerging from the gas burner 40, and a flame sensor 56which is also of a conventional type and which serves to check that thegas/air mixture emerging from the gas burner 40 has been correctlyignited by the ignition device 54. The purpose of the flame sensor 56will be described in more detail below. (The electrical ignition device54 and the flame sensor 56 may alternatively be combined into a singledevice which performs both functions.)

The combustion products produced by burning of the gas/air mixturewithin the combustion chamber 38 pass through a heat exchangercomprising a riser section 58 which is connected to the upper end of,and extends vertically upwardly from the frusto-conical section 36 ofthe combustion chamber member, and a helical section 60, which extendsdownwardly from the upper end of the riser section 58. The riser section58 lies along the axis of the tank 12 and the axis of the helicalsection 60 coincides with the axis of the tank 12. It will be noted thatthe riser section and the coil are of substantially uniformcross-section from beginning to end, preferably about one andthree-quarter inches in diameter O.D. (about 4.4 cm.). It has been foundthat this form of heat exchanger provides a very efficient heat transferfrom the combustion products to the water in the tank 12, because thedownward slope of the helical section 60 contributes to a highercoefficient of convective heat transfer (since the coolest downstreamsection of the heat exchanger is in contact with the coolest wateradjacent the base of the tank 12) and therefore to higher efficiency ofthe heat exchanger. It should be noted that the lower end of the helicalsection 60 lies adjacent the base 24 of the tank 12; since water heatedby the heat exchanger tends to rise within the tank 12, it is desirablethat the lower end of the heat exchanger extend adjacent the base of thetank 12 in order to ensure that a large mass of cold water is not leftbelow the heat exchanger and remains unheated thereby. The combustionproducts leaving the lower end of the helical section of the heatexchanger pass through an exhaust conduit having a first verticalsection 62 extending vertically downwardly from the lower end of theheat exchanger 60 through the base 24 of the tank 12, a short horizontalsection 64 which extends into the insulating blanket 16, and a second,long vertical section 66 which extends vertically upwardly from thehorizontal section 64 through the insulating blanket 16 and through anaperture provided in the upper end plate 18. The riser 58 and coil 60(about fourteen inches, about thirty-five cm., in outside diameter)having an O.D. of about one and three-quarters inches has a lengthbefore exiting the tank of about thirty feet (about nine meters). Theindicated length and diameter combined with a four inch (ten cm.) waterguage impeller pressure and a gas feed rate corresponding to about80,000 Btu/hr. results in condensation of combustion products in thelower turns of the coil in a thirty to thirty-eight gallon tank (orobviously any larger size tank). The condensation temperature ofcombustion products is about 140° F. and feed water from normal watersupplies is about 55° to 60° F. As a result of the condensation theefficiency of the system is increased because the heat of condensationis given up to the coil. Because combustion products inherently includea mild carbonic acid, it is preferred that the coil be constructed of439 or 444 stainless steel. Specifically, the condensate will containsmall quantities of CO₂,NO and NO₂ and in combination with the condensedH₂ O, the condensate will comprise a mildly corrosive acid.

From the upper end of the second vertical section 66, a section 68carries the combustion products out of the building in which theapparatus is installed. (The form of the section 68 may vary for thereasons already stated in relation to the corresponding section of theinlet tube 48, and similarly the vertical section and/or section 68 maybe omitted entirely.) The placement of the second vertical section 66within the insulating blanket 16 serves to prevent accidental damage tothe second vertical section, as already described in relation to the airinlet tube 48. Under ordinary circumstances, section 68 will be inclinedslightly toward the tank 12 so that any condensation will drain back toa drain conduit 74 which will be explained subsequently. It isundesirable to have a mildly corrosive acid (which results fromcondensation) drain onto the ground and this could be the result ifsection 68 is not inclined toward tank 12.

A water supply line 70 used to supply water to the tank 12 passesthrough an aperture provided in the upper end plate 18 and then passesvertically downwardly within the second vertical section 66 of theexhaust conduit, along the first horizontal section 64 of this conduitand then vertically upwardly through an aperture provided in the base 24of the tank 12 into the interior of that tank. The disposition of thewater supply line 70 within the sections 66 and 64 of the exhaustconduit allows heat exchange to take place between the combustionproducts passing along the exhaust conduit and the water entering thetank via the water supply line, thus pre-warming the water entering thetank 12 and further cooling the combustion products, thereby increasingthe efficiency of the apparatus. At the point where it enters theexhaust conduit, the water supply line is provided an U-bend 72; thisU-bend reduces the tendency for water warmed by heat transfer from thecombustion products passing along the exhaust conduit from rising intothe portion of the water supply line 70 lying above the upper end plate18. Such back-flow of water into the part of the water supply line 70lying above the plate 18 is obviously undesirable since it leads to heatloss from the apparatus, thus reducing the efficiency thereof. Inaddition, the resultant leakage of hot water into the cold water systemmay produce undesirable changes in temperature therein.

The cooling of the combustion products which occurs in the heatexchanger 58, 60 by heat transfer to water in the tank 12, and in theexhaust conduit by heat transfer to the water in the water supply line70, causes condensation of liquid from the combustion products as statedpreviously, and the resultant condensate passes into the firsthorizontal section 64. Thus, the U-shape formed by the sections 62, 64and 66 serves as a condensate trap. A drain conduit 74 extendsdownwardly from the first horizontal section 64 and serves to draincondensate therefrom. The drain conduit 74 is provided with a U-bendbelow the first horizontal section 64; this U-bend rapidly becomesfilled with condensate when the apparatus is in operation, and thecondensate filling the lowest part of the U-bend acts as a liquid sealto prevent gaseous combustion products flowing out through the drainconduit 74. The outlet end (not shown) of the drain conduit 74 isconnected to a suitable drain line. Because of the aofrementionedacidity of the condensed combustion products, it is preferred that thedrain 74 and perhaps exhaust section 62, 64, 66 be of plastic such asPVC.

As already mentioned, the apparatus 10 substitutes for both a waterheater and a furnace. A water outlet line 76 extends from within thetank 12 through the upper end wall of the tank and through the upper endplate 18. This water outlet tube 76 serves to supply hot water fordomestic use, as indicated schematically by a branch line. Anotherbranch line leaves the water output line 76 a short distance above theupper end plate 18 and serves to supply hot water from the tank 12 to aspace heating assembly comprising a coil 82 disposed within a duct 84,which forms part of the ductwork of a conventional forced-air domesticspace heating system. Air is forced over the coil 82 by means of athermostatically-controlled fan 86 (shown only schematically). Waterfrom the coil 82 is returned to the tank via a return line 88, whichintersects the water supply 70 a short distance above the upper endplate 18. As will be apparent to those skilled in the art, the apparatus10 can also be used in conjunction with a baseboard radiator or othertype of hot water space heating system.

The dimensions of the various parts of the apparatus 10 can of coursevary considerably depending upon various factors, and in particular uponthe desired thermal output of the apparatus. For example, a 100,000Btu/hr. (25,200 KCal/hr.) apparatus for domestic use may have a tank 12having a capacity of about forty gallons (151 liters) and approximatelysixteen inches (45 cm.) in diameter. In steady state operation, such aunit would use one hundred cubic ft. (2.83 m³) of gas per hour andapproximately 1,100 cubic ft. (31.1 m³) of air per hour, assuming thatthe amount of air is set at the optimum value of approximately 10% abovethat stoichiometrically required for combustion of the gas. This gas/airmixture will be forced by the impeller 46 at a pressure of approximatelyfour inches (10 cm.) water gauge pressure into the internal chamber ofthe gas burner 40. The cylindrical portion of the gas burner 40 has alength of approximately four inches (10 cm.) and a diameter ofapproximately 2.2 inches (5.5 cm.). The uppermost three inches (7.5 cm.)of the cylindrical wall of the gas burner 14 is pierced by apertureshaving a diameter of 0.033-0.036 inches (0.84-0.91 mm.) (though theapertures could vary within the range of 0.020-0.062 inches (0.50-1.57mm.) if desired), spaced on a square grid at intervals of approximately0.070 inches (1.78 mm.); the lines within this grid run parallel to theaxis of the cylindrical gas burner 40 and on circles running around thegas burner.

It has been found that a 100,000 Btu/hr. unit constructed having theforegoing dimensions has a steady-state efficiency in excess of 95% andexperiments indicate that the seasonal efficiency or service efficiencyof the apparatus would be up to about 93%. The combustion productsleaving the apparatus have a low nitrogen oxide content, probably due tothe near stoichiometric combustion comditions within the combustionchamber 38 made possible by the control of the gas/air ratio which theapparatus provides. The apparatus does not need a chimney or a class Aor B vent; the outlet from the exhaust conduit may be discharged throughcorrosion resistant tubing through any convenient external surface ofthe building in which the apparatus is installed. It should be notedthat the combined steady-state efficiency of water and space heating ofabout 95% is greater than that of any combined system available.

It will of course be appreciated that if desired the heating coil 82,the duct 84 and the fan 86 could be replaced by one or more conventionalwater-filled radiators for space heating purposes, a forced-air systembeing shown in the drawing simply because this is the most common typeof system used in domestic space heating. Obviously, if water-filledradiators are to be used in space heating, it will be necessary toprovide the water outlet line 76 or the return line 88 with acirculating pump to effect circulation of water through the radiators.(In practice, although not shown in the accompanying drawing, acirculating pump will in practice usually be necessary in forced-airsystems also.)

All the essential features of the apparatus 10 have already beendescribed. However, in order to ensure the safest possible operationunder domestic conditions, it is desirable that a number of safetydevices be incorporated into the apparatus to cope with possibleequipment failures and unusual operating conditions. As alreadymentioned, the flame sensor 56 is provided within the combustion chamber38 to detect an absence of flame due either to failure of the ignitiondevice 54 or to failure to supply proper quantities of gas because ofimproper operation of the impeller 46. When the flame sensor detects anabsence of flame while the impeller 46 should be operating, the flamesensor 56 is arranged to open a switch (not shown) which shuts down theimpeller 46. Also, the air inlet line 48 is provided with a pressuresensor (not shown) adjacent the inlet to the impeller 46. When theimpeller is operating normally, significant suction exists in the airinlet conduit 48 and if inadequate suction exists in this line, theimpeller is not operating correctly and accordingly the pressure sensoris arranged to shut the impeller down. The pressure sensor is alsoarranged to shut the impeller down if excessive suction exists in theline 48 as a result of, for example, obstruction of the air intake tothis line by debris or other materials.

FIG. 2 is an alternative embodiment and the prime difference is theelimination of the plumbing in the walls of the tank. It will beobserved that the air feed line 48 terminates in a stub at the surfaceof the outer housing in the area between the flange 30 and the baseplate 20. The same is true of the exhaust conduit 64 and the drain line26 and water feed line 70. The reason for this is to prevent piping frombeing exposed to shipping and installation damage. The requiredconnections and associated conduit assembly will be made on the job sitebecause each job site requires a unique structural arrangement.

It will be observed that the valve 50 of FIG. 1 has been eliminated fromFIG. 2 to illustrate the concept of a negative pressure valve discussedearlier.

The downardly extending end 62 of the stainless steel coil 60 is shownentering a PVC collar 63 which connects in turn to an L-shaped PVCextension 64.

The water supply line 70 of FIG. 2 is connected to the return line 88 inessentially the same way as in FIG. 1 in the sense that each isillustrated schematically. A pump 90 is shown in line 88 and itsindicated location is preferred, in that, it sould be on the downstreamside from heat exchanger 82 because of the lower temperature of thewater being pumped. The pump 90 will have a longer life where it pumpsthe lower temperature water.

A pressure relief valve 92 is shown schematically in line 76 and such isconventional.

Those skilled in the art will appreciate that, although the specificembodiment of the invention described above is intended for use as acombined water/space heating system, the instant apparatus may also beuseful as a high-efficiency water-heating system without withoutspace-heating capability, especially in commercial water heatingsystems.

It will be apparent to those skilled in the art that numerous changesand modifications can be made in the embodiments of the inventiondescribed above without departing from the scope of the invention.Accordingly, the foregoing description is to be construed in anillustrative and not in a limitative sense, the scope of the inventionbeing defined solely by the appended claims.

I claim:
 1. A method for burning a combustible gas comprising:mixingsaid combustible gas with an oxygen-containing gas in a conduit to forma combustible mixture capable of supporting combustion without additionof extra oxygen-containing gas; controlling the volume flow of thecombustible gas to said conduit in proportion to the pressure of theoxygen-containing gas, passing said combustible mixture under pressureinto an internal chamber of a gas burner, said gas burner having wallsdefining said internal chamber and apertures passing through said walls,said apertures being sized so that combustion of said combustiblemixture outside said gas burner will not cause ignition of saidcombustible mixture within said internal chamber of said gas burner;permitting said combustible mixture to pass through said apertures;burning said combustible mixture as it leaves said apertures to formcombustion products, discharging the combustion products to a heatexchanger, said heat exchanger being located in a tank of liquid;cooling the combustion products within the heat exchanger to the extentthat some of said combustion products condense; and discharging saidliquid and gaseous combustion products from said exchanger through anexhaust conduit.
 2. A method according to claim 1 wherein said burningof said combustible mixture is effected with a closed combustion chamberhaving no gas inlet other than said gas burner.
 3. A method according toclaim 2 wherein said combustion chamber is immersed in liquid.
 4. Amethod according to claim 2 wherein said heat exchanger includes a risersection extending upwardly from said combustion chamber and a helicalsection extending downwardly from the upper end of said riser section.5. A method according to claim 2 wherein said combustion products passto a condensate trap which separates said liquid from the remainder ofsaid combustion products.
 6. A method according to claim 1 wherein saidgas burner is substantially cylindrical in form and said apertures aredisposed in the cylindrical wall of said gas burner so that said burningof said combustible mixture takes place on a substantially cylindricalflame front.
 7. Apparatus for burning a combustible gas comprising;ahousing having walls defining a liquid chamber capable of holdingliquid; a combustion chamber member disposed within said liquid chamberand having liquid-impervious walls defining the combustion chamber; anexhaust conduit having an inlet connected to said combustion chamber andan outlet through which combustion products can leave said apparatus; agas burner mounted within said combustion chamber, said burner havingwalls defining an internal chamber and apertures passing through saidwalls, thereby establishing fluid communication between said internalchamber of said gas burner and said combustion chamber outside said gasburner; an impeller for passing a combustible mixture of saidcombustible gas and an oxygen-containing gas under pressure into saidinternal chamber of said gas burner, said apertures in said gas burnerbeing sized such that combustion of said combustible mixture within saidcombustible chamber outside said gas burner will not cause ignition ofsaid combustible mixture within said internal chamber of said gasburner, a duct having an air inlet and an air outlet; means for movingair through said duct from said air inlet to said air outlet; a firstheat exchanger disposed within said duct for effecting heat exchangebetween hot water passing through said heat exchanger and air passingthrough said duct around said heat exchanger, said heat exchanger havinga water inlet and a water outlet; a water supply conduit having an inletdisposed within said liquid chamber and an outlet connected to saidwater inlet of said heat exchanger; a water return conduit having aninlet connected to said water outlet of said heat exchanger and anoutlet disposed within said liquid chamber, said water return conduithaving a section mounted in parallel to a section of said exhaustconduit and configured to cause the combustion products passing alongsaid exhaust conduit and the water flowing along said water returnconduit to flow counter-current to one another and to cause heat to flowfrom said combustion products to said water; and an outer housingsurrounds said housing, at least part of said exhaust conduit isdisposed between said housing and said outer housing, and part of saidliquid return conduit is disposed within said exhaust conduit.
 8. Theapparatus of claim 7 including,a second heat exchanger in fluidcommunication with said combustion chamber means and encompassed withinsaid liquid chamber for receiving the combustion products generated bysaid fuel burner and for effecting heat exchange between said combustionproducts and liquid in said liquid chamber; said exhaust conduit havingan inlet in fluid communication with said second heat exchanger forreceiving said combustion products and an outlet through which saidcombustion products can leave said apparatus.
 9. The apparatus accordingto claim 8 wherein said exhaust conduit comprises a first sectionextending downwardly from the lower end of said helical section of saidsecond heat exchanger, a second section extending upwardly from thelower end of said first section, and a drain conduit extendingdownwardly from adjacent the junction of said frst and second sections.10. Apparatus according to claim 7 wherein an outer housing surroundssaid housing, at least part of said exhaust conduit is disposed betweensaid housing and said outer housing, and part of said water returnconduit is disposed within said exhaust conduit.
 11. Apparatus accordingto claim 8 wherein said second heat exchanger comprises a riser sectionextending upwardly from said combustion chamber and a helical sectionextending downwardly from the upper end of said riser section, andwherein said exhaust conduit comprises a first section extendingdownwardly from the lower end of said helical section of said secondheat exchanger, a second section extending upwardly from the lower endof said first section, and a drain conduit extending downwardly fromadjacent the junction of said first and second sections.
 12. Incombination: a heating duct, a water tank, a combustion chamber withinsaid tank, and a burner within said chamber;means for supplying amixture of fuel and oxygen under pressure to said burner; means forconducting hot water from the tank to a first heat exchanger mountedwithin said heating duct; means for returning water from said first heatexchanger to said tank; a second heat exchanger mounted within saidtank, said second heat exchanger being in fluid communication with saidcombustion chamber for receiving combustion products discharged fromsaid chamber; an exhaust conduit in fluid communication with the secondheat exchanger for discharging combustion products to the atmosphere; acold water inlet conduit to the tank, said water conduit and saidexhaust conduit being mounted in parallel and configured to (1) causethe combustion products and water to flow countercurrent to each otherand (2) to cause heat to flow from the combustion product to the coldwater; and an outer cylinder encompassing the tank and chamber, aninsulating jacket in the space between the tank and the cylinder, thecountercurrent flowing conduits being located within said insulatingjacket.
 13. The combination of claim 12 including means for joining thewater flow from the cold water inlet conduit with that of the returningwater from the first heat exchanger into a single conduit anddischarging water from said single conduit into said tank.
 14. Thecombination of claim 13 wherein the single conduit is configured to bein heat exchange relationship with said exhaust conduit.
 15. Thecombination of claim 14 wherein said means for supplying a mixture offuel and oxygen comprises an air conduit and a fuel conduit joinedtogether with an impeller, said impeller being in fluid communicationwith said burner,means for controlling the ratio of fuel and air in saidmixture in proportion to the pressure in said air conduit.
 16. thecombination of claim 13 wherein said means for supplying a mixture offuel and oxygen comprises an air conduit and a fuel conduit joinedtogether with an impeller, said impeller being in fluid communicationwith said burner,means for controlling the ratio of fuel and air in saidmixture in proportion to the pressure in said air conduit.
 17. thecombination of claim 12 wherein said means for supplying a mixture offuel and oxygen comprises an air conduit and a fuel conduit joinedtogether with an impeller, said impeller being in fluid communicationwith said burner,means for controlling the ratio of fuel and air in saidmixture in proportion to the pressure in said air conduit.
 18. Apparatusfor burning a combustible gas comprising;a housing having walls defininga liquid chamber capable of holding liquid; a combustion chamber memberdisposed within said liquid chamber and having liquid-impervious wallsdefining the combustion chamber; an exhaust conduit having an inletconnected to said combustion chamber and an outlet through whichcombustion products can leave said apparatus; a gas burner mountedwithin said combustion chamber, said burner having walls defining aninternal chamber and apertures passing through said walls, therebyestablishing fluid communication between said internal chamber of saidgas burner and said combustion chamber outside said gas burner; animpeller for passing a combustible mixture of said combustible gas andan oxygen-containing gas under pressure into said internal chamber ofsaid gas burner, said apertures in said gas burner being sized such thatcombustion of said combustible mixture within said combustion chamberoutside said gas burner will not cause ignition of said combustiblemixture within said internal chamber of said gas burner, a duct havingan air inlet and an air outlet; means for moving air through said ductfrom said air inlet to said air outlet; a first heat exchanger disposedwithin said duct for effecting heat exchange between hot water passingthrough said heat exchanger and air passing through said duct aroundsaid heat exchanger, said heat exchanger having a water inlet and awater outlet; a water supply conduit having an inlet disposed withinsaid liquid chamber and an outlet connected to said water inlet of saidheat exchanger; a water return conduit having an inlet connected to saidwater outlet of said heat exchanger and an outlet disposed within saidliquid chamber, said water return conduit having a section mounted inparallel to a section of said exhaust conduit and configured to causethe combustion products passing along said exhaust conduit and the waterflowing along said water return conduit to flow countercurrent to oneanother and to cause heat to flow from said combustion products to saidwater; and said exhaust conduit comprises (1) a riser section extendingupwardly from said combustion chamber, (2) a helical section extendingdownwardly from the upper end of said riser section, (3) a first sectionextending downwardly from the lower end of said helical section, (4) asecond section extending upwardly from the lower end of said firstsection, and (5) a drain conduit extending downwardly from adjacent thejunction of said first and second sections.
 19. The apparatus accordingto claim 8 wherein an outer housing surrounds said housing, at leastpart of said exhaust conduit is disposed between said housing and saidouter housing, and part of said liquid return conduit is disposed withinsaid exhaust conduit.